The architecture of computing systems has undergone tremendous changes in the recent past, due principally to the advance of microcomputers from the original four-bit chips running at hundreds of kilohertz to the most modern 32 and 64 bit microprocessors running at hundreds of megahertz. As the chip designers push to higher and higher speeds, problems arise which relate to thermal issues. That is, as the speed of a circuit is increased, the internal logic switches must each discharge its surrounding capacitance that much faster. Since the energy stored in that capacitance is fixed (at a given voltage), as the speed is increased, that energy, which must be dissipated in the switches, is dumped into the switch that many more times per second. Since energy per second is defined as power, the power lost in the switches therefore increases directly with frequency.
On the other hand, the energy stored in a capacitance increases as the square of the voltage, so a capacitor charged to two volts will store only 44% of the energy stored in that same capacitor charged to three volts. For this reason, a microcomputer designed to operate at two volts will, when run at the same speed, dissipate much less power than the same microprocessor operating a three volts. So there is a tendency to lower the operating voltage of microprocessors.
Other considerations cause the microprocessor to exhibit a lower maximum speed if operated at a lower voltage as compared to a higher operating voltage. That is, if a circuit is operating at full speed, and the voltage on that circuit is simply reduced, the circuit will not operate properly, and the speed of the circuit (the “clock speed”) may have to be reduced. To maintain full speed capability and still operate at lower voltage, the circuit often must be redesigned to a smaller physical size. Also, as the size of the circuitry is reduced, and layer thickness is also reduced, the operating voltage may need to be lowered to maintain adequate margin to avoid breakdown of insulating oxide layers in the devices. For the past few years, these steps have defined the course of microprocessor design. Key microprocessor designers, seeking the maximum speed for their products, have therefore expended considerable effort trading off the following considerations:
higher speed chips are worth more money;
higher speed chips must dissipate more heat;
there are limitations to removal of that heat;
lower voltages reduce the heat generated at a given speed; and
smaller devices run faster at a given voltage.
Of course, there are many, many important trade-off considerations beyond these, but the above list gives the basic elements which relate to some aspects of the current invention. The result of these considerations has been for the microprocessor designers to produce designs that operate at lower and lower voltages. Early designs operated at five volts; this was reduced to 3.3. to 3.0, to 2.7, to 2.3, and at the time of writing the leading designs are operating at 2.0 volts. Further reductions are in store, and it is expected that future designs will be operated at 1.8, 1.5, 1.3, 1.0, and even below one volt, eventually perhaps as low as 0.4 volts.
Meanwhile, advances in heat removal are expected to permit processors to run at higher and higher heat dissipation levels. Early chips dissipated perhaps a watt; current designs operate at the 30 watt level, and future heat removal designs may be able to dissipate as much as 100 watts of power generated by the processor. Since the power dissipated is proportional to the square of the operating voltage, even as the ability to remove heat is improved, there remains a tendency to run at lower operating voltages.
All of this is driven by the fundamental consideration: higher speed chips are worth more money. So the designers are driven to increase the speed by any and all means at their disposal, and this drives the size of the chips smaller, the voltages lower, and the power up. As the voltage drops the current increases for a given power, because power is voltage times current. If at the same time improvements in heat removal permit higher powers, the current increases still further. This means that the current is rising very rapidly. Early chips drew small fractions of an ampere of supply current to operate, current designs use up to 15–50 amperes, and future designs may use as much as 100 amperes or more.
As the speed of the processors increase, the dynamics of their power supply requirements also increase. A processor may be drawing very little current because it is idling, and then an event may occur (such as the arrival of a piece of key data from a memory element or a signal from an outside event) which causes the processor to suddenly start rapid computation. This can produce an abrupt change in the current drawn by the processor, which has serious electrical consequences.
Inductance is the measure of energy storage in magnetic fields. All current-carrying conductors have associated with their current a magnetic field, which represents energy storage. It is well known by workers in the art that the energy stored in a magnetic field is half the volume integral of the square of the magnetic field. Since the field is linearly related to the current in the conductor, it may be shown that the energy stored by a current carrying conductor is proportional to half the square of the current, and the constant of proportionality is called the “inductance” of the conductor. The energy stored in the system is supplied by the source of electrical current, and for a given power source there is a limit to the rate at which energy can be supplied, which means that the stored energy must be built up over time. Thus the presence of an energy storage mechanism naturally slows down a circuit, as the energy must be produced and metered into the magnetic field at some rate before the current can build up.
The available voltage, the inductance, and the rate of change of current in a conductor are related by the following equation, well known by those skilled in the art:V=L*∂I/∂t, where L is the inductance of the conductor, and ∂I/∂t is the rate of change of current in the conductor.This equation states that the voltage required to produce a given current change in a load on a power system increases as the time scale of that change is reduced, and also increases as the inductance of any connection to that load is increased. As the speed of microprocessors is increased, the time scale is reduced, and as the available voltage is reduced, this equation requires the inductance to be dropped proportionally.
Normally, in powering semiconductor devices one does not need to consider the inductance of the connections to the device, but with modern electronics, and especially with microprocessors, these considerations force a great deal of attention to be brought to lowering the inductance of the connections. At the current state of the art, for example, microprocessors operate at about two volts, and can tolerate a voltage transient on their supply lines of about 7%, or 140 millivolts. These same microprocessors can require that their supply current change at a rate of at least one-third or even nearly one ampere per nanosecond, or 3*108 or 109 amperes/second, respectively. The above equation indicates that an inductance of about 140 picohenries (1.4*10−10 H) and ½ nanohenry, (5*10−10 H) will drop a voltage of 140 millivolts at these two rates of current rise. To put this number in perspective, the inductance of a wire one inch in length in free space is approximately 20 nanohenries, or 20,000 picohenries. While the inductance of a connection can be reduced by paralleling redundant connections, to create a connection with an inductance of 140 picohenries with conductors about a centimeter long would require some 20 parallel conductors, and for instance a connection with an inductance of ½ nanohenry would require nearly 100 parallel conductors.
The foregoing discussion implies that the source of low voltage must be physically close to the microprocessor, or more generally the active portion of a particular component, which in turn implies that it be physically small. While it might be suggested that capacitors might be used to supply energy during the delay interval required for the current in the conductors to rise, the intrinsic inductance of the connections to the capacitors currently severely limits this approach. So the system designer is faced with placing the source of power very close to the processor to ensure that the processor's power source is adequately stable under rapid changes in current draw. This requirement will become increasingly severe as the voltages drop still further and the currents increase, because the former reduces the allowable transient size and the latter increases the potential rate of change of current. Both factors reduce the permissible inductance of the connection. This can force the designer to use smaller capacitors which have low inductance connections, and because the smaller capacitors store less energy, this drives the power system to higher frequencies, which adds costs and lowers efficiency.
The foregoing remarks are not limited in computers to the actual central microprocessor. Other elements of a modern computer, such as memory management circuits, graphic display devices, high speed input output circuitry and other such ancillary circuitry have been increased in speed nearly as rapidly as the central processing element, and the same considerations apply.
Many modern electronics circuitry, including computers, are powered by switchmode power conversion systems. Such a system converts incoming power from the utility line to the voltages and currents required by the electronic circuitry by operation of one or more switches. In low power business and consumer electronics, such as desktop personal computers, the incoming power is supplied as an alternating voltage, generally 115 volts in the United States, and 220 volts in much of the rest of the world. The frequency of alternation is either 50 or 60 Hertz, depending upon location. Such utility power must be converted to low voltage steady (direct) current, or dc, and regulated to a few percent in order to be useful as power for the electronic circuits. The device which performs such conversion is called a “power supply”. While it is possible to create a low voltage regulated DC power source using simple transformers, rectifiers, and linear regulators, such units would be heavy, bulky and inefficient. In these applications it is desirable to reduce weight and size, and this approach is unsuitable for this reason alone. In addition, the inefficiency of linear regulators is also unacceptable. Efficiency is defined as the ratio of output power to input power, and a low efficiency implies that heat is being developed in the unit which must be transferred to the environment to keep the unit cool. The lower the efficiency the more heat must be transferred, and this is itself a reason for finding an alternate approach.
For these reasons, virtually all modern electronics circuitry is powered by switchmode conversion systems. These systems typically operate as follows. The incoming utility power is first converted to unregulated direct current by a rectifier. The rectified DC is then converted to a higher frequency, typically hundreds of kilohertz, by electronic switches. This higher frequency power is then transformed by a suitable transformer to the appropriate voltage level; this transformer also provides isolation from the utility power, required for safety reasons. The resulting isolated higher frequency power is then rectified again, and filtered into steady direct current for use by the electronics. Regulation of the output voltage is usually accomplished by control of the conduction period of the electronic switches. The resulting power conversion unit is smaller and lighter in weight than earlier approaches because the size and weight of the transformer and output filter are reduced proportionally to the increase in frequency over the basic utility power frequency. All of this is well known in the prior art.
In a complex electronic system, various voltages may be required. For example, in a computer system the peripherals (such as disk drives) may require +12 volts, some logic circuits may require +5 volts, input/output circuits may additionally require −12 volts, memory interface and general logic may require 3.3 volts, and the central microprocessor may require 2 volts. Standards have developed so that the central power source (the device that is connected directly to the utility power) delivers ±12 and +5 volts, and the lower voltages are derived from the +5 supply line by additional circuitry, called voltage regulation modules, or VRMs, near to the circuits that require the lower voltage. These additional circuits convert the +5 volt supply to high frequency AC power again, modifying the voltage through control of the period of the AC power, and again re-rectifying to the lower voltage dc.
The resulting overall system is complex and not very efficient, in spite of the use of switchmode technology. In a typical 200 watt computer system, four watts are lost in the initial rectification of the utility line, eight watts in the electronic switches, 2.5 watts in the transformer, 20 watts in the output rectification and filtering, and four watts in the connections between the center power supply and the electronics boards. Thus 38.5 watts are lost in the conversion process for the higher voltage electronic loads. Substantial additional losses may be sustained in the low voltage conversion process. A typical 50 watt voltage regulation module, which may convert +5 volts at 10 amperes to +2 volts at 25 amperes for the microprocessor, will itself have losses of about one watt each in the AC conversion and transformer, and ten watts in the final rectification and filtering. Other voltage regulation modules will have losses almost as great, resulting in losses for the entire system which may be one-third of the power used. Some particularly inefficient approaches may demonstrate efficiencies as low as 50%, requiring that the input power circuits utilize twice the power required by the actual final circuitry, and requiring that twice the heat be dissipated in the electronics (which must be removed by a fan) as is theoretically required by the actual operating circuitry.
This system evolved over the years and is not optimum for many current uses, but persists because of inertia of the industry and because of the perceived benefit of maintaining industry standards on voltages and currents as generated by the central power unit.
An analysis of current trends in the microprocessor industry clearly indicates that the current system will not be adequate for the future. These trends show that the current draw of critical elements such as the core microprocessor has been steadily increasing and will continue to do so into the future. Meanwhile, the operating voltage has been steadily decreasing, dropping with it the allowable tolerance of the supply voltage in absolute terms. Finally, the rate of change of processor current—the current slew rate—is increasing very rapidly, with substantial additional increases forecast for the near future. All of these factors mitigate against the current technology and require a new approach to be adopted in the future. It has been reliably estimated that the current powering and other technology will not last more than one additional generation of microprocessors, and since designers are currently at work on the generation following the next, it can be said that these designers are in the process of developing a microprocessor which cannot be powered by currently available technology.
A further problem in the prior art is the use of square wave electronic conversion techniques. Such technology, known as PWM, for Pulse Width Modulation, produces switch voltage waveforms which have steeply rising edges. These edges produce high frequency power components which can be conducted or radiated to adjacent circuitry, interfering with their proper operation. These high frequency power components may also be conducted or radiated to other electronic equipment such as radio or television receivers, also interfering with their proper operation. The presence of such components requires careful design of the packaging of the power system to shield other circuitry from the high frequency power components, and the installation of expensive and complex filters to prevent conduction of these components out of the power supply package on its input and output leads. What is needed then, is a power conversion system which enables small, highly efficient voltage regulation modules to be placed close to the point of power use, which is fast overall, and which is itself efficient and at least as low in cost as the prior art technology it replaces.